This project deals with both laboratory and clinical aspects of infection caused by the intestinal nematode, Strongyloides stercoralis. The laboratory work is now focused upon preparation and purification of recombinant antigens from infective (L3) larvae of the parasite. The clinical studies involve assay and evaluation of immune responses in individuals infected with S. stercoralis. The diagnosis of chronic Strongyloides infection is often difficiult because larval excretion is scanty and intermittent. Therefore, a serologic test and other immunologic methods are useful in identifying infected individuals. Assay of antibodies of different isotypes, such as IgG, IgG4 and IgE, has been found useful in immunodiagnosis. An immediate hypersensitivity skin test mediated by parasite-specific IgE using crude parasite antigens continues to be studied. The effect of infection with the retrovirus, HTLV-1, upon individuals also infected with S. stercoralis was found to affect the likelihood of severe disease due to the parasite and efficacy of treatment. The immunologic mechanism for this was found to be due to in vivo activaton of interferon-gamma by HTLV-1 and consequent downregulation of IL-4 which results in reduction of serum IgE levels. This immunologic mechanism is being studied by analysis of lymphocyte expression of intracellular cytokines.